X-ray tube anodes emit only a fraction of the incoming radiated energy in the form of X-rays. The rest is converted into heat and has to leave the anode in the form of thermal radiation.
It has therefore been known for many years to improve the thermal emissivity of X-ray anodes made of refractory metals by means of an oxide coating (as described, for example, in Austrian Patent Specification 337 314, German Offenlegungsschrift 22 01 979, German Offenlegungsschrift 24 43 354). These prior disclosures separately describe and lay claim, by means of various oxide materials and manufacturing processes, to increase the adhesion of the oxide layer to the surface of the parent metal, as compared with the prior art, and to raise the thermal emissivity of the anode surface. However, it has been found that the performance of layers produced as described in the prior disclosures is limited in view of the continuously increasing requirements imposed on X-ray anodes with regard to layer ageing, heat radiating capability and also resistance to degassing (i.e., the avoidance of electrical flashovers).
German Offenlegungsschrift 22 01 979 describes, in particular, an oxide layer which is composed of a heating product containing titanium dioxide and additions of at least one other refractory oxide. Other suitable oxides are mentioned, such as aluminum oxide, calcium oxide, magnesium oxide and zirconium oxide. No mention is made of particular advantages experienced by any particular or special oxide combination. From the examples and the subordinate claims, it is to be concluded that a mixture of approximately equal parts of aluminum oxide and titanium dioxide is a preferred oxide mixture. In addition, it is to be concluded from the description that it is important for the titanium dioxide content not to drop below 20%.
EU A2 0 172 491 describes, in a further development, an X-ray anode made of a molybdenum alloy having an oxide coating composed of a mixture of 40%-70% titanium oxide, with the remainder being stabilizing oxides from the group comprising ZrO.sub.2, HfO, MgO, CeO.sub.2, La.sub.2 O.sub.3 and SrO. In order to improve the achievement of the oxide layers with respect to the above-mentioned requirements imposed upon the layers, this prior disclosure sets as a particular object fusing the oxides to form smooth, brightly lustrous layers by means of economical processes.
EU A2 0 244 776 essentially relates to the same inventive subject. The invention relates to the pretreatment of the oxide material, before its application to the X-ray anode by means of standard spraying techniques. In this case, a mixture composed of 77%-85% titanium dioxide containing 15%-23% by weight of calcium oxide is processed in a first step to produce a powder containing a homogeneous phase. This mixture is then applied to the anode, being optionally mixed with other oxide powders, by known spraying processes. The reference describes various coating processes, such as plasma-jet spraying, sputtering processes, chemical and physical gas-phase deposition processes or even the electron beam process, as suitable for oxide coating of X-ray anodes made of refractory metals. Normally, an X-ray anode made of refractory metal is then subjected to a degassing annealing at the end of the production process. The degassing annealing of the anode serves to avoid gas emissions and, as a consequence thereof, highly undesirable plasma flashovers between the electrodes when the latter are used in high vacuum in an X-ray tube.
The inventive teaching of this prior disclosure implies that it is advantageous to match the material composition of the oxide layer to the annealing treatment applied after coating the X-ray anodes. These degassing annealings simultaneously serve for final formation and fusing of the oxide phase, i.e., the conversion of the oxide phase to a state which cannot be achieved solely by an oxide application process, such as the plasma-jet spraying process.
However, the layer composition according to this prior disclosure and the processes for producing it only inadequately meet the requirements imposed. On the contrary, during the annealing of the oxide layers according to this prior disclosure, there is the risk that, at an annealing temperature at which the oxides fuse to form smooth, well-adhering layers, the latter are already so fluid that the contour between coated and uncoated parts of the X-ray anode surface becomes ill-defined to an undesirable extent. Such ill-defined contours cannot be tolerated, in particular, in the region of the focal track.
In addition, such oxide layers experience troublesome gas phase formation at the required annealing temperatures.
U.S. Pat. No. 4,870,672 describes an oxidic coating for X-ray anodes, composed of a mixture of Al.sub.2 O.sub.3, ZrO.sub.2 and TiO.sub.2. The preferred composition of the coating is composed of 40-70% by weight of TiO.sub.2, 20-40% by weight of ZrO.sub.2, and 10-20% by weight of Al.sub.2 O.sub.3. The limit compositions of the coating are specified as 10-80% by weight of TiO.sub.2, 10-60% by weight of ZrO.sub.2, and 5-30% by weight of Al.sub.2 O.sub.3. A disadvantage in the case of this coating is that, with an unfavorable choice of composition, evaporation of the coating may occur, with consequent condensation and flashover in the X-ray tube.